1. Field of the Invention
The present invention relates to a power supply device for a wire electric discharge machine capable of suppressing wire electrode consumption.
2. Description of the Related Art
According to a power supply device for a conventional electric discharge machine, machining current is maintained based on a current (electric energy) stored in an inductor the moment a switching element is turned off after the power supply voltage is applied between an electrode and a workpiece when electric discharge is caused. The current stored in the inductor does not have a constant value because of resistance components and arc voltage and is reduced with time. Thereafter, switching is performed to regenerate the current in the inductor to the power supply, thereby quickly stopping current flow to the workpiece.
A technique disclosed in Japanese Patent Application Laid-Open No. 11-48039 uses two dc power sources, a high-voltage main DC power source and a low-voltage auxiliary DC power source. Power supply voltages from these two dc power sources are applied simultaneously, and discharge current is rapidly started by the main DC power source immediately after the start of electric discharge. Thereafter, the voltage application from the main DC power source is stopped, and the discharge current is kept constant based on current from the auxiliary DC power source.
In an electric discharge apparatus, the peak value of electric discharge machining current is proportional to the period of voltage application. High energy should be input to increase the machining speed. The higher the peak, the higher the machining speed is. If the peak is higher, however, then wire electrode consumption will be greater.
According to the prior art technique, the wire electrode consumption and load on a filter are so heavy that the filter life is short. Thus, the filter must be replaced so frequently that the running cost increases. If the peak of pulses of the electric discharge machining current is lowered, the machining speed is inevitably reduced, although the wire electrode consumption can be suppressed.